Method for the fabrication of electrolyte cavities using bulk micro-machining

ABSTRACT

The present invention is directed to the fabrication of thin aluminum anode batteries using a highly reproducible process that enables high volume manufacturing of the galvanic cells. A method of fabricating a thin aluminum anode galvanic cell is provided, the method comprising, forming a recess in the silicon wafer, the recess having no more than three sidewalls, depositing a catalytic metal layer on a bottom surface of the recess, positioning a double-side sticky tape layer having a bottom side positioned to contact the no more than three sidewalls of the recess and positioning an aluminum foil layer to contact a top side of the double-side sticky tape layer and in overlying relation to the recess, thereby forming the galvanic cell.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with Government support under Grant No.DASG60-00-C-0089 awarded by the U.S. Army Space and Missile DefenseCommand. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

Aluminum galvanic cells, also commonly known as fuel or semi-fuel cells,provide mechanical systems. System integration of power sources of thiskind makes possible the development of sensors that can be deployed inthe field.

One of the most claimed advantages of MEMS systems is the low energyrequirement. So, these MEMS fabricated cells with higher energeticcapacities can potentially power several of these micro-systemscomponents, enabling the development of complex sampling schemes. Thecells can be activated on demand, which eliminates the time-degradationperformance common to commercial available batteries. The cells can bedisposable (depending on the actuation type) and can be activated ondemand and can provide a very long on-the-shelf life. System integrationof both portable and disposable analytical/sensing systems benefit fromthis simple power source.

Fabrication methods for these aluminum galvanic thin cells are known inthe art. However, the prior art methods utilize unsophisticatedmanufacturing techniques, such as staking and gluing layers. Thesetechniques are difficult to duplicate in a mass fabrication environment

Accordingly, what is needed in the art is a highly manufacturableprocess for the fabrication of high-energy micro-aluminum galvaniccells.

SUMMARY OF INVENTION

In accordance with the present invention, a method to fabricate thinaluminum anode batteries using a highly reproducible process thatenables high volume manufacturing of the galvanic cells is described.

In accordance with a particular embodiment, semiconductor fabricationmethods are used to fabricate the thin aluminum galvanic cells inaccordance with the present invention. In a particular embodiment, acatalytic material to be used as the cathode is deposited on asubstrate. An insulating spacing material is deposited on the cathodeand patterned using photolithography. This insulating spacing materialcan be either used as a sacrificial layer to expose the electrodes orserve as a support for one of the electrodes. Similarly, the aluminumanode may be deposited and patterned on another substrate and bonded tothe first substrate, or can be deposited directly on the insulatingmaterial prior to patterning. The cell is packaged using standardtechniques and connected to a delivery system to provide delivery of theelectrolyte when activation of the cell is desired.

In a particular embodiment, a thin aluminum anode galvanic cell isfabricated on a silicon wafer having a catalytic metal layer positionedon a patterned silicon substrate to form an electrode mesh, a dielectriclayer positioned to cover the catalytic metal mesh layer and an aluminumlayer positioned to cover the dielectric layer. In a specific embodimentof this galvanic cell, the catalytic metal is platinum and the galvaniccell further includes a titanium nitride adhesive layer positionedbetween the patterned silicon substrate and the platinum layer. In aspecific embodiment, the platinum layer is about 100 nm thick, thetitanium nitride layer is about 10 nm thick, and the aluminum layer isabout 300 nm thick.

In an additional embodiment, a thin aluminum anode galvanic cell on asilicon wafer is provided having a recess formed in the silicon wafer,the recess having three sidewalls, a catalytic metal layer positioned onthe bottom of the recess and an aluminum foil layer positioned inoverlying relation to contact the three sidewalls of the recess, therebyforming the galvanic cell. In a particular embodiment of this galvaniccell, the catalytic metal layer is a platinum layer, and a titaniumnitride layer is positioned between the silicon wafer and the platinumlayer. In a specific embodiment, the platinum layer is about 100 nmthick, the titanium nitride layer is about 10 nm thick and the aluminumfoil layer is about 100 μm thick.

In yet another embodiment, a thin aluminum anode galvanic cell isprovided having a first substrate comprising a catalytic layer and a BCBlayer, the BCB layer forming the walls of a reservoir on the surface ofthe catalytic layer and a second substrate comprising an aluminum layerand bonded to the first substrate such that a galvanic cell is formedbounded by the catalytic layer, the aluminum layer and the BCB layerforming the walls of the reservoir.

The thin batteries manufactured in accordance with the methods of thepresent invention are capable of delivering a substantial amount ofenergy for their size, per unit weight when compared to largerbatteries, during specific intervals of time. While the aluminumgalvanic cells do not provide as high of a voltage as other systemsknown in the art, such as lithium cells, they are capable ofconsiderably higher current densities. Larger voltages can be achievedif high currents are available. Additionally, the materials used in themanufacturing process of the cells are environmentally inert materials.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference should be made tothe following detailed description, taken in connection with theaccompanying drawings, in which:

FIG. 1 is an illustration of a galvanic cell in accordance with thepresent invention.

FIG. 2 is a diagrammatic view fabricated meshed cells in accordance withthe present invention.

FIG. 3 is a diagrammatic view of fabricated cells withelectrolyte-cavities manufactured via bulk micro-machining in accordancewith the present invention.

FIG. 4 is a diagrammatic view fabricated cells with electrolyte-cavitiesmanufactured via surface micromachining in accordance with the presentinvention.

FIG. 5 is a graphical representation of the achieved thicknesses aftersoft-bake for patterned BCB. Error bars are generated as plus minustwice the standard deviation

FIG. 6 is a graph illustrating exemplary data at different loads for themeshed cells in accordance with the present invention, including 1 Kohm, 500 ohm 200 ohm and 100 ohms.

FIG. 7 is a graph illustrating the potential as a function of time for 4different chemistry recipes tested in accordance with the presentinvention. Results for cells with platinum to aluminum area ratio of 4are illustrated.

FIG. 8 is a graphical illustration of the potential as a function oftime for 4 different chemistry recipes tested in accordance with thepresent invention. Results for cells with platinum to aluminum arearatio of 9 are illustrated.

FIG. 9 is a graphical illustration of the cell potentials as a functionof time at various loads for 1 cm² micro-aluminum galvanic cells inaccordance with the present invention

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings, which form a partthereof, and within which are shown by way of illustration specificembodiments by which the invention may be practiced. It is to beunderstood that other embodiments may be utilized and structural changesmay be made without departing from the scope of the invention.

In an exemplary embodiment, a fabrication method for fabricating verythin (MEMS) aluminum anode galvanic cells in accordance with the presentinvention is described. The fabrication steps are based on, but notlimited to, conventional semi-conductor industry methodologies.

With reference to FIG. 1, an exemplary embodiment to fabricate the thinaluminum anode batteries using a highly reproducible process thatenables high volume manufacturing of the galvanic cells is illustrated.In accordance with this embodiment, semiconductor fabrication methodsare used to fabricate the thin aluminum galvanic cells in accordancewith the present invention. In a particular embodiment, a catalyticmaterial to be used as the cathode 60 is deposited on a substrate 75. Aninsulating spacing material 55 is deposited on the cathode 60 andpatterned using photolithography. This insulating spacing material 55can be either used as a sacrificial layer to expose the electrodes orserve as a support for one of the electrodes. Similarly, the aluminumanode 50 may be deposited and patterned on another substrate and bondedto the first substrate 75, or can be deposited directly on theinsulating material 55 prior to patterning. In this exemplaryembodiment, the silicon nitride passivation layer 70 and adhesion layer65 are illustrated. The adhesion layer 65 may be titanium nitride,aluminum nitride, silicon, or silicon dioxide. The cell is then packagedusing standard techniques and connected to a delivery system to providedelivery of the electrolyte when activation of the cell is desired.

In an exemplary embodiment illustrating the fabrication process inaccordance with the present invention is provided. A first step in thefabrication process of the galvanic cell involves the deposition of ametal, on a substrate forming the cathode collector of the galvaniccell. In a particular embodiment, platinum is used as the depositingmetal and a silicon wafer is used as the substrate. However, it iswithin the scope of the present invention to utilize other metals withdifferent degrees of efficiency. Silver, for instance provides similarresults to platinum, nickel, copper and other known catalytic metals canbe used only limited by the reactivity with the chosen electrolyte.Next, aluminum metal is deposited to constitute the cells' anodes. Thismetal can be deposited on a substrate through various methods,including, but not limited to, e-beam evaporation, and plasma physicalvapor deposition. Alternatively a thin aluminum plate or foil can beused as an anode. Both anode and cathode substrates should provide meansfor collecting the electric current. With this embodiment, the anode andcathode separation ranges from submicron to micron dimensions (MEMSbatteries). The cells may additionally be fabricated in a meshedconfiguration in which the cells are built vertically on the substrate.

The coating of a dielectric (ceramics, or polymeric materials are suitedfor this purpose) that separates the cell's electrodes is furtherincluded within the scope of the invention. This separating material canbe first coated to either one of the substrates or simply placed inbetween the electrodes and be able to bond at contact or by eitherapplying pressure or temperature.

The optional step of patterning the deposition of the metals to thesubstrates, either via sacrificial etching or lift off techniques, orthe separating dielectric is considered and exemplified here. Animportant result from the patterning of the metals arises from seeking amore efficient chemical reaction resulting in more energy provided permass or volume of reagents. The bonding of the layers using a patternedseparating material forms a micro-fluidic reservoir that will containthe electrolyte. However, in an additional embodiment, batteries can befabricated and a reservoir patterned substrate layer can be subsequentlybonded to them.

In accordance with the present invention, three particular embodimentsfor producing the micro cells are presented. These methods include: (1)Fabrication Process for the Production of Meshed Cells, (2) FabricationProcess for Cells with Electrolyte-cavities via bulk micro-machining,and (3) Fabrication Process for Cells with Electrolyte-cavities viasurface micromachining.

Exemplary Embodiment (1) Fabrication Process for the Production ofMeshed Cells

In a particular embodiment of the invention, a process for theproduction of meshed cells is provided. With reference to FIG. 2, inaccordance with this embodiment, the process begins with <100>, 100 mmdiameter silicon wafers 80. The crystal orientation and dopingconcentration is inconsequential to device operation. The wafers 80 werecleaned with acetone, followed by methanol and deionized water and spundry. A low stress, 300 nm thick silicon nitride layer 85 was thendeposited in a Tystar LPCVD furnace (Torrance, Calif.), at 835° C. for60 minutes with NH₃ flow rate of 20 scm/min. Negative photoresist(NR9-1000PY, acquired from Futurrex, Inc, Franklin, N.J.) was then spunonto the wafer at 3000 RPM for 40 seconds, followed by a 150° C.pre-bake for 1 minute. Using a mask created to pattern the platinumelectrodes and EVG alignment system, the photoresist was exposed for 11seconds, followed immediately by a post exposure bake at 100° C. for 1minute. After allowing the wafer a small amount of time to cool, theresist was developed in RD-6 photoresist developer. After rinsing andspinning dry, the wafer was subjected to another minute at 100° C. todrive out any moisture associated with the development process.

Using an AJA International model ATC 1800 sputtering system, a 10 nmtitanium nitride layer 90 was deposited. In an exemplary embodiment,deposition time was 3 minutes with RF power set at 360 W, argon flowrate at 29.4 scm/min, nitrogen flow at 2.6 scm/min and chamber pressureset at 2 mTorr. It is shown that this portion of the process can bemodified to test different adhesion layers. Alternative adhesion layerssuccessfully tested were silicon and aluminum nitride.

Following the deposition of the adhesion layer 90, a 100 nm-thick layerof platinum 95 was deposited using the in-house sputtering system for 4minutes. Flow rate was 30 scm/min for argon at a pressure of 2 mTorr.Liftoff of the platinum 95 was performed using 1165 photoresist stripperat 80° C., to pattern the active battery areas. Liftoff times were 15minutes for the wafers with the silicon adhesion layers, 20 minutes forthe wafer with the titanium nitride adhesion layer 90 and over 90minutes for the wafer with aluminum nitride. Upon completion of theliftoff process, the wafer was cleaned with isopropanol and deionizedwater and spun dry.

An alternative to the lifttoff process is an etching processing (As itwill be explained for aluminum patterning in the 2^(nd) type of cells:Electrolyte reservoir cells). This type of processing requires thedeposition of the metals prior to applying and developing photoresist.Either a negative image of the mask used, or a positive resist would berequired to properly pattern the electrodes. The wafers would be placedin platinum etchant for the amount of time recommended by themanufacturer. If a conductive adhesion layer is used, this would requirean additional etch step, followed by the appropriate rinse and spin dry.

Next, a 500 nm silicon dioxide layer 85 was deposited in a Unaxis(Osaka, Japan) 790 PECVD (plasma enhanced chemical vapor deposition)system using a standard recipe for oxide deposition. Substratetemperature was 250° C. and deposition occurred for 10.5 minutes. Thesubstrate was cooled to 150° C. prior to removing from the chamber.Cells have also been fabricated using silicon nitride as this dielectriclayer using the same equipment (with different gases) as described forthe silicon dioxide layer.

Using the in-house sputtering system, aluminum 100 was deposited. Again,argon flow rate was 30 scm/min. Deposition time was 30 minutes. Aluminumthickness was measured to be 300 nm±10% after the etch step using aprofilimeter. Following aluminum deposition, positive photoresist(Shipley 1813) was spun onto the wafer at 3000 RPM for 40 seconds,followed by a 1 minute bake at 90° C. The pattern was aligned andexposed for 1.8 seconds, then developed for 40 seconds in MF 319. Thewafer was then rinsed in deionized water and spun dry. The aluminum wasetched in aluminum etchant for 8 minutes, constantly agitated. Uponcompletion of the etching step, the wafer was rinsed with deionizedwater and spun dry. The photo resist was then removed using Shipley 1165photoresist stripper at 80° C. for 8 minutes, followed by rinse withisoproponal and deionized water and spun dry. Additionally, thephotoresist can be removed by spinning at 3000 rpm for 40 seconds withacetone followed by methanol.

Alternatively, aluminum 100 can be deposited using an electron beamevaporation system, producing thicker layers of aluminum for a givenprocessing time.

The oxide was etched in a Unaxis 790 RIE (reactive ion etching) systemusing a standard recipe for oxide etching. Etch time was 13 minutes. Theplatinum electrode acted as an etch stop and the aluminum electrodeacted as a mask, preserving the oxide directly under the aluminumelectrode. The resulting devices had a total electrode area of 1 cm²each.

Exemplary Embodiment (2) Fabrication Process for Cells withElectrolyte-Cavities Via Bulk Micro-Machining

The process begins with <100>, 100 mm diameter silicon wafers 105,(Siltron Inc. Korea). The crystal orientation and doping concentrationis inconsequential to device operation. The wafers 105 were cleaned withacetone, followed by methanol and deionized water and spun dry, (aspinner model WS-400B-6NPP/Lite acquired from Laurell Corp, North West,Pa., was used). In this case a recess 110 is formed in the silicon wafer80. At the bottom of this recess 110 platinum 115 is deposited andaluminum foil 125 is subsequently glued to the wafer via a double sidesticky tape 130, as shown with reference to FIG. 3.

The recess 110 is formed via etching, while silicon nitride is used asthe mask. The first step is then the deposition of a low stress, 300 nmthick silicon nitride layer was deposited in a Tystar LPCVD furnace(Torrance, Calif.), at 835° C. for 60 minutes with NH3 flow rate of 20scm/min. Once the silicon nitride layer covers the whole wafer thismaterial is patterned using a Laser micromachining tool. Alternatively,the nitride can be patterned and etched either chemically or in a plasmaetcher.

Chemical etching is used to form the recess 110 on the silicon wafer 80.A 45% solution of potassium hydroxide at 80 C of temperature is used.The immersion in this solution lasted for approximately 25 minutes. Thetotal height is about 100 um. Once the recess 110 is formed a platinum“liftoff” process deposits a 100-um thick layer on the bottom of therecess. The word liftoff is in quotation marks as a traditional photoresist was not employed. Instead, kapton tape is placed around theformed recess 110. As an alternative to the chemical etching and siliconnitride mask, a standard photo-resist can be used in conjunction with aRIE or DRIE machine with the appropriate recipes to create the recess.

Then the platinum deposition takes place. First a 10 nm-thick titaniumnitride layer 120 was deposited to improve the platinum adhesion to thesilicon wafer using an AJA International (North Scituate, Mass.). modelATC 1800 sputtering system. Deposition time was 3 minutes with RF powerset at 360 W, argon flow rate at 29.4 scm/min, nitrogen flow at 2.6scm/min and chamber pressure set at 2 mTorr. Subsequently, a 100nm-thick layer of platinum 115 was deposited using the in-housesputtering system for 4 minutes using an in-house built sputteringsystem. The tape is removed, leaving the metals only in the cavity 110.

High purity (99.999%) 100-um thick aluminum 125 (acquired from AlfaAesar) is cut into pieces, with dimensions such that 1 cm² of aluminumarea is directly exposed to the platinum. Contact was made to the Pt andAl electrodes using metallic pins, now shown. Electrolyte was introducedto the cavity 110 using a syringe.

Exemplary Embodiment (3) Fabrication Process for Cells withElectrolyte-Cavities Via Surface Micromachining

In accordance with this embodiment as shown with reference to FIG. 4,platinum 130 was deposited on single side polished prime, 4inch-diameter, 500 um-thick silicon wafers 140. Aluminum 145 isdeposited on 4 inch-diameter, 500 um-thick, glass wafers 155 to allowfor visualization of the cell at the time of filling. Cells have alsobeen fabricated using silicon wafers as a substrate on which thealuminum anodes are patterned. The aluminum 145 and platinum 130deposition conditions are similar to those described in the meshedcells. For both cases titanium nitride 135, 150 is used as the adhesionlayer. A 100 nm thick layer of platinum 130 was deposited on one of thewafers 140. After the platinum 130 is deposited, a small area of thewafer is cut (or cleaved) on the side. This small cut on the platinumwafer will offer an exposed area of the aluminum, which will serve as anelectrical contact point.

Aluminum 145 was deposited using a PVDX 1800 evaporation system (AJAInternational, North Scituate, Mass.). The deposition took place at apressure of 4×10⁻⁶ torr at a 7.0 kV potential and a current of 480mamps. A 25 minute-long deposition was employed to obtain a 1.4 um-thickhigh purity aluminum deposit. Puratronic© 4-8 mm aluminum (purity of99.999%) shot acquired from Alfa Aesar (Ward Hill, Mass.) was used. Thealuminum 145 is etched to cover an area slightly smaller than thatencircled by the benzocyclobutene layer (BCB) 160, to ensure a goodliquid sealed, as shown in FIG. 4. This helps prevent the delaminationof the cell when the aluminum etches away upon cell activation. Thepatterning of the aluminum was done by selective etching. Positive photoresist (S1813, from Shipley Co, Freeport N.Y.), was used. The resist isapplied on the wafer, spun at 3000 rpm for 40 second and the hot-platebaked for 1 minute. The photo-resist is then exposed in an EV 620aligner (EVG, Schärding, Austria) for 2.5 seconds. MF 319 is used asdeveloper. A 30-minute-long immersion of the patterned wafer wasnecessary to etch completely the aluminum in the exposed areas. Type-Aaluminum etchant (Transene Co, Danvers, Mass.) at room temperature wasused to pattern the aluminum.

The BCB 160 procedure basis for patterning was guided by themanufacturer recommendations [www.cyclotene.com]. The BCB (Cycloteneresin 4026 was acquired from Dow Chemical Co.) was deposited andpatterned on the platinum side. The wafers' surfaces are cleaned byapplying acetone immediately followed by methanol application while thewafers are rotating to free the Platinum surface from any organicresidues and particles. The spinning was performed in aWS-400B-6NPP/Lite spinner (Laurell Corp, North West, Pa.). Then adhesionpromoter is applied (AP3000) statically while wafer on spinner; spreadit by lightly moving the chuck. The promoter is allowed to interact withthe wafer for 30 seconds. The wafers are then spun-dry at 3000 rpm for15 seconds. The wafers with the AP3000 residue were then baked at 125°C. for 30 seconds. The BCB is applied on the wafer, while the wafer ismotionless on the chuck. The resin is spread by rotating the spinner at700 rpm during approx 10 seconds. The spinner speed was varied dependingon the desired BCB thickness for 30 seconds. Achieved thicknesses aftersoft-bake for patterned BCB are shown with reference to FIG. 5. Errorbars are generated as plus minus twice the standard deviation

After BCB-resin is applied a hot-plate bake (Delta 20T2 3LE) wasperformed to partially harden the resin before exposure. Cyclotene is aphoto-negative resin so light-field masks were prepared to expose thepolymer areas that are to form the walls of the reservoirs. Masks weredrawn using Coventor software and printed in transparency millard paper(at Precision Images, Largo, Fla.). The resin was allowed to cool downbefore exposure. Exposure is a function of the film thickness. For 4026resin, the manufactorer's recommended exposure dose is 60 mJ/cm2/um. AnEV-620 photo lithographic system was used that provided an irradiance of15 mW/cm2. Before developing an oven soft-bake was recommended. It wasfound that the development process was greatly enhanced by performingthis oven (Ultra clean 100 In Line Instruments) bake for 10 minutes. Thetemperatures are shown below in table 1.

TABLE 1 Pre-bake and pre-develop bake temperatures for deposited BCB ofvarious thicknesses. After softbake thickness, μm 4.6- 6.7- 8.8- 10.1-11.5- <4.6 6.6  8.7  10  11.4  15.6  >15.6 Hot-plate prebake 60 65 70 7580 85 90 temperature, ° C. Oven predevelop 50 55 60 65 70 75 80 baketemperature, ° C.

The wafers were then developed while in movement using the puddledevelopment technique using DS2100 as the developer. Wafers were placedon spinner and while spinning (700 rpm), developer was added drop bydrop during 10 seconds, followed by an immediate spun dried at 3000 rpmfor 30 seconds. This procedure is repeated several times. When spinningat 1500 rpm, which results on a BCB 160 thickness of 20 um, thedevelopment procedure is repeated five times. A spinning speed of 3000rpm produces a BCB 160 thickness of approximately 10 um and requiresonly repetition of the development procedure three times. Wafers arethen placed in a plasma etcher (M4L Tepla) to remove the remainingpolymer covering the wafer's clear areas exposing the metals. Aone-minute etching time was sufficient for the processing proceduredescribed above. The etching gas recipe used kept the Dow's recommendedratio of 80:20 O2/CF4.

The bonding of the processed wafers was then performed using a EV501(EVG, Schärding, Austria) wafer-to-wafer bonder. The platinum wafer 140with the patterned BCB 160 is placed on the wafer holder. Thealuminum-wafer 155 is then coated with adhesion promoter as done withthe platinum wafer as described above. The aluminum wafer is then placedon top of this one, making sure that the wafers are aligned so a portionof the metals are easily access to serve as electrical contacts.Alternatively, alignment marks can be used with the EV-620 alignmentsystem to align the wafers. In the case of the bonding of two siliconwafers the holes 165 previously made serve to make sure the ports wouldpermit the electrolyte filling. In the case of the glass mask it iseasily to optically align the masks.

Flags are placed to separate the wafers. The bonding chamber is thenclosed and the sequence is initiated. The following steps comprise thebonding sequence: (1) The vacuum pump extracts the air from the chamberuntil a pressure of 1×10⁻² mbar is reached. The top and bottom platesare then heated to a temperature of 160° C., and it is held for 5minutes. This step is to minimize overshoot for the next step. Then moreheating until the plates are brought up to 205° C. is required. Thistemperature is held there during 30 minutes. This long period of time isnecessary, as this temperature brings the polymer to a temperature atwhich it softens eliminating the possibility of formation of smallcracks in the polymers surface. After that the wafer is bow (startsbonding from the center), the flags are taken out and the piston is helddown using a force of 900 N. The system is then purged to a pressure of5×10⁻⁴ mbars. This low pressure ensures that low concentrations ofoxygen, which inhibites the polymerization reaction, are present whilethe resin cures. The wafers are then heated up to 230° C. and held therefor 30 minutes to ensure the curing of the resin.

The metal deposition and combination of patterning and bondingoptimization on the wafers using BCB as described are exemplary innature. Other combinations are within the scope of the presentinvention.

In an additional embodiment, ports for injecting electrolyte into thecells are provided to enable the activation of the galvanic cellsutilizing an appropriate electrolyte.

In accordance with the present invention, the cell fabrication methodspresented ensure that the electrodes distance is very small, thusminimizing overpolarizations due to ion transport. This has allowed usto use simple electrolyte formulations. No extra additives have beenutilized in the formulations for which the corresponding data ispresented, but this possibility may further enhance the cellperformance.

Exemplary data resulting from the fabricated cells in accordance withthe present invention are illustrated with reference to FIG. 6-FIG. 9.

It will be seen that the advantages set forth above, and those madeapparent from the foregoing description, are efficiently attained andsince certain changes may be made in the above construction withoutdeparting from the scope of the invention, it is intended that allmatters contained in the foregoing description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween. Now that theinvention has been described,

What is claimed is:
 1. A method of fabricating a thin aluminum anodegalvanic cell on a silicon wafer, the method comprising: forming arecess in the silicon wafer, the recess having no more than threesidewalls; depositing a catalytic metal layer on a bottom surface of therecess; positioning a double-side sticky tape layer having a bottom sidepositioned to contact the no more than three sidewalls of the recess;and positioning an aluminum foil layer to contact a top side of thedouble-side sticky tape layer and in overlying relation to the recess,thereby forming the galvanic cell.
 2. The method of claim 1, whereinprior to forming a recess in the silicon wafer, further comprisingcleaning the substrate with acetone, followed by methanol and thendeionized water, and spinning the wafer until dry.
 3. The method ofclaim 1, wherein forming a recess in the silicon wafer further comprisesforming a recess in the wafer using etching with silicon nitride as amask.
 4. The method of claim 1, wherein forming a recess in the siliconwafer further comprises: depositing a silicon nitride layer on the waferat about 835° C. for 60 minutes with NH₃ flow rate of about 20 scm/min;patterning the silicon nitride layer; and chemical etching the patternedsilicon nitride layer to form the recess on the silicon wafer.
 5. Themethod of claim 4, wherein patterning the silicon nitride later furthercomprises patterning the layer using a laser micromachining tool.
 6. Themethod of claim 4, wherein patterning the silicon nitride layer furthercomprises patterning the layer using chemical or plasma etching.
 7. Themethod of claim 4, wherein chemical etching the patterned layer furthercomprises chemical etching the wafer using about a 45% solution ofpotassium hydroxide at about 80° C. for about 25 minutes.
 8. The methodof claim 1, wherein forming a recess in the silicon wafer furthercomprises forming a recess having a depth of about 100 μm in the wafer.9. The method of claim 1, wherein the deposited catalytic metal layer isplatinum that is about 100 μm thick.
 10. The method of claim 1, whereindepositing a catalytic metal layer on a bottom surface of the recessfurther comprises the steps of: placing kapton tape around the formedrecess; depositing a layer of titanium nitride within the recess;depositing a layer of catalytic metal over the titanium nitride layer;and removing the tape, thereby leaving the deposited metals in therecess.
 11. The method of claim 10, wherein the step of depositing thetitanium nitride layer further comprises depositing the titanium nitridelayer by sputtering for about 3 minutes with RF power set at about 360W, argon flow rate at about 29.4 scm/min, nitrogen flow at about 2.6scm/min and the sputtering chamber set at about 2 mTorr.
 12. The methodof claim 10, wherein the step of depositing the catalytic metal layerfurther comprises depositing a layer of platinum by sputtering for about4 minutes.
 13. The method of claim 1, wherein the aluminum foil layer isabout 100 μm thick.
 14. The method of claim 1, further comprisingcontacting the catalytic metal and aluminum electrodes using metallicpins.
 15. The method of claim 1, further comprising introducing anelectrolyte into the recess of the galvanic cell.